Property and Equipment, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Cost of property and equipment	$ 1,052	$ 979
Less accumulated depreciation	(770)	(685)
Property and equipment, net	282	294	$ 258
Depreciation expense	138	130	121
Rental expense	75	71	$ 65
Land
Property, Plant and Equipment [Line Items]
Cost of property and equipment	1	1
Buildings
Property, Plant and Equipment [Line Items]
Cost of property and equipment	4	4
Leasehold improvements
Property, Plant and Equipment [Line Items]
Cost of property and equipment	248	224
Computer equipment
Property, Plant and Equipment [Line Items]
Cost of property and equipment	700	658
Office furniture and other equipment
Property, Plant and Equipment [Line Items]
Cost of property and equipment	$ 99	$ 92